1. Technical Field
The present invention relates to a fuel cell system and, more particularly, to the structure of a fuel cell system that facilitates cooling of a stack of the fuel cell system.
2. Description of the Related Art
A fuel cell is a system for producing electric power. In a fuel cell, energy resulting from a chemical reaction between oxygen and hydrogen contained in hydrocarbon-group materials, such as methanol, ethanol, and natural gas, is directly converted into electric energy. A unique characteristic of the fuel cell is that both the electricity generated by the electrochemical reaction between fuel and an oxidizing agent (without involving a combustion process) and its by-product heat may be utilized.
Depending on the type of electrolyte used, a fuel cell is classified into different types as follows: a phosphate fuel cell operating in a range of approximately 150˜200° C.; a molten carbonate fuel cell operating in a range of approximately 600˜700° C.; a solid oxide fuel cell operating at 1000° C. and above; and a polymer electrolyte or alkali fuel cell operating in a range between room temperature and 100° C. Although each of these different types of fuel cells operates using the same principles, they differ in the type of fuel, catalyst and electrolyte used, as well as in their drive temperature.
A polymer electrolyte membrane fuel cell (PEMFC) is presently undergoing development. Compared to other fuel cells, the PEMFC has excellent output characteristics, a low operating temperature, and fast starting and response characteristics. The PEMFC has a wide range of applications, including use in vehicles through utilization of hydrogen made by reforming methanol, natural gas, etc., use in the home and in buildings, and use as a power source in electronic devices.
The basic components of the PEMFC include a stack, a reformer, a fuel tank, and a fuel pump. The stack forms the main body of the fuel cell. The fuel pump supplies fuel in the fuel tank to the reformer. The reformer reforms the fuel during the process of supplying the fuel stored in the fuel tank to the stack, thereby generating hydrogen gas, and then supplies the hydrogen gas to the stack. Accordingly, the PEMFC sends the fuel in the fuel tank to the reformer by operation of the fuel pump, the reformer reforms the fuel to generate hydrogen gas, and the hydrogen gas undergoes an electrochemical reaction with oxygen in the stack, thereby generating electric energy.
Another type of fuel cell is the direct methanol fuel cell (DMFC). The DMFC differs from the PEMFC in that liquid methanol is directly supplied to the stack using this method such that there is no need for a reformer.
In the above fuel cell systems, the stack (where the generation of electricity takes place) is structured so as to include a few to a few tens of unit cells, each comprising a membrane electrode assembly (MEA) and separators (or bipolar plates) provided on both sides thereof. In the MEA, an anode electrode (also referred to as a “fuel pole” or “oxidation electrode”) and a cathode electrode (also referred to as an “air pole” or “reduction electrode”) are provided in opposition to one another with an electrolyte layer interposed therebetween. The separator functions to provide a pathway through which hydrogen gas and oxygen, which are required for fuel cell reaction, are supplied to the anode electrode and the cathode electrode of the MEA. In addition, the separator functions as a conductor for connecting the anode electrode and cathode electrode of each MEA in series. Accordingly, fuel gas containing hydrogen is supplied to the anode electrode, and oxygen gas containing oxygen is supplied to the cathode electrode via the separator. Through this process, electrochemical oxidation of the fuel gas occurs in the anode electrode, and electrochemical reduction of the oxygen gas occurs in the cathode electrode. Electricity is generated by the movement of electrons which occurs during this process. Heat and moisture are also generated.
In the above fuel cell systems, the stack must be constantly maintained at a suitable temperature in order to ensure stability of the electrolyte layer, as well as to prevent a reduction in overall performance. To realize this, an air-cooling type cooling apparatus is typically used in such a system to cool the stack by blowing air of a relatively low temperature thereon, and then exhausting the resulting heated air.
However, a drawback of the fuel cell system having the air-cooled type cooling structure, as described above, is that the air that is heated by passing through the stack is simply discarded. This is a significant waste of energy.
In addition, only part of the air supplied to the cathode electrode of the MEA through the separator for electricity generation of the stack is reacted, while the rest of this air is unreacted and exhausted in a state containing a large amount of moisture. When this air containing a large amount of moisture is exhausted from the stack to the atmosphere of a relatively low temperature, condensation is generated through contact with the atmosphere. Therefore, additional devices are required to store or re-use the water generated through this process. This increases the size of the system. If the additional device requires operation, heat is generated through this process and electricity consumed, thereby reducing the overall efficiency and performance of the fuel cell system.